Software transactional memory (STM) is a concurrency control mechanism analogous to database transactions for controlling access to shared memory in concurrent computing. A transaction in the context of transactional memory is a piece of code that executes a series of reads and writes to shared memory. A data value in the context of transactional memory is the particular segment of shared memory being accessed, such as a single object, a cache line (such as in C++), a page, a single word, etc.
A software transactional memory system must implement transactional semantics for all memory operations, otherwise atomicity and isolation are compromised. Object-based STMs use a per-object concurrency control mechanism: each object contains “metadata” used by the STM for concurrency control (a kind of lock). This requires that, at the point of invoking a transactional operation on a field of an object, the object's identity can be discovered. But even object-based systems have data not stored in objects: e.g., static and local variables. STMs may transct such data in a different fashion. Unfortunately, in a software system that permits taking an address of a data item and passing this address to separately compiled functional units, often the source of the data item cannot be recovered. As an example, consider this C# code:
class Clss {  int m_fld;  static int s_fld;}void a( ) {  int j = 7;  Data d = new Clss( );  int[ ] arr = new int[1];  b(ref j);  b(ref d.m_fld);  b(ref Clss.s_fld);  b(ref arr[0]);}void b(ref int x) {  atomic {    x++;  }}
This code examples above illustrate the classic problem. When method b is compiled, the runtime argument that will be supplied for the parameter x is not known. Yet the caller, a, calls b with four different types of values, each of which refers to a type of location which utilizes different concurrency control mechanisms. Generally, object-based STM systems ensure transactional semantics for static variables, local variables (locals and arguments), and instance fields or array elements in different ways, meaning b must somehow recover the source of the argument x.